Also, an anti-riot police officer whose visor was hit by a small metal ball was injured, police said.

Police labeled the group as “rioters,” which carries a heavy prison sentence if convicted.

The campus is in the city’s Hung Hom district, just across Victoria Harbor from Hong Kong Island and close to major roads, including across harbor tunnel. Also near the Chinese People’s Liberation Army base.

Protests which included disrupting the city’s public transportation by blocking the roads, lasted from Sunday morning into Monday local time.

Near the university on a flyover at the toll booths at the Cross-Harbour Tunnel, a police armored vehicle caught fire from gas bombs thrown by protesters, local broadcaster TVB reported. The fire was put out by police tear gas fired to disperse the protesters.

From balconies, protesters used catapults to launch bricks and other hard objects at police lines from the university’s balconies.

“Such attacks pose a grave threat to the safety of police officers, reporters and first aiders at the scene,” said police in a statement, accusing protesters of firing petrol bombs and metal balls at their lines.

Police attempted to disperse the group with tear gas and water canons.

“They showed total disregard for the safety of everyone at scene,” police said.

PolyU urged students, staff and alumni to evacuate the campus, public broadcaster RTHK reported. The university said dangerous chemicals were missing from its laboratories.

The Hong Kong Professional Teachers’ Union and the university’s Staff Association said they were “very worried” about the safety of students and bystanders.

“We call on both sides to exercise restraint and avoid the use of deadly weapons. The current stalemate is caused by a series of government decision-making mistakes and should be resolved responsibly,” the statement said.

Protesters originally took to the streets in marches against a controversial China extradition bill, which would have sent offenders to the mainland. Although the government suspended pursuit of the bill, the movement’s focus now includes complaints of police brutality and wider calls for democracy.

“If we don’t come out, no one will come out and protect our freedoms. Polytechnic University is my home,” A 23-year-old protester and Polytechnic University alumni told CNN Sunday.

The city’s Education Bureau announced that classes at all schools will remain suspended on Monday. Since Thursday, classes at kindergartens, primary and secondary schools as well as special schools have been suspended for their safety,

In addition, the Social Welfare Department advised people not to take their children to child care and after-school care programs Monday. Also people are advised not to go to elderly and rehabilitation centers.